A NOTE ON GENERALIZED DERI VATION RANGES

Transcription

1 Chin. Am. of Math. 6B (3) 1985 A NOTE ON GENERALIZED DERI VATION RANGES L u 8н 1 Л Е ( - # Д ) * Abstract This paper is devoted to the generalized derivations determined by compact operators acting on Banach spaces. The author introduces the concept o f fam ily o f approxim ately linearly indepedent vectors and em ploys this concept to prove the follow ing result: Let X, У be Banach spaces, А = В (Х ), B B(Y) be compact operators. Then S~AB has closed range i f and only if both A and В have closed range. Let X, Y be Banach spaces over complex field О (throughout, we assume dim X = d im F = oo). Let B ( X, Y ) denote the set of all bounded linear operators from X to Y. If X = F, we write B (X ) for B (X, Y ). For A B (X ) and B B (Y ), the generalized derivation ЗГAB is an operator belonging to B (B (Y, X ) ) defined by th e equation If X = Y and A = B = T, derivation 3TiBS = M - S B, < y )S B (Y,X ). the generalized derivation is reduced to the inner AiS = T S - S T, (V)fl B ( X ). Similarly, as in the case of inner derivation, one of the problems on generalized derivations is: 'under what conditions on A and B, is the range of fyab norm closed?. In 1975 J. Anderson and 0. Foias ш proved that if A and В are scalar operators acting on Banach spaces X, Y respectively, then ZTAB has closed range if and only if Л=0 is an isolated point of сг(^и*). In 1980, L. A. Fialkow 12-1 gave some conditions under which 2ГAB has closed range where A, В are hyponormal operators, nilpoteht operators of order 2 and compact operators in a Hilbert space respectively. The aim of this note is to prove the following Theorem. Let X, Y be Banach spaces and A B ( X ), B B (Y ) be compact operators, then STAB has closed range i f and only i f both A and В have closed range. In order to prove this theorem we give some notation first. N will be the set / Manuscript received April 6, Bevised Saptember 13, 1983, * Department of Mathematics, Nanjing Univevsity, Nanjing, China.

2 262 CHIN. ANN. OF MATH. Vol. 6 Ser. В of natural numbers. For a Banach space X, X* will denote the conjugate space of X. For a subset M czx, M 1 will denote the annihilator of Ж ж х = {»*ех*, <», *>=о, (v) g j o, S P{M} will denote the subspace spanned by Ж S P{M} = c losure«igo, coi M, ng-vj. For an operator T B ( X, Y ), ker T, R (T ) will denote the null space and the range of T respectively. nult will denote the nullity of T, i. e., the dimension of кегз7. nul't will denote the approximate nullity of T, i. e., the greatest num ber m < o o with the following property: for any s> 0, there exists an m dimensional closed linear manifold M Bsuch that \\Txl <8 a? for every x Жв (see [3], p. 232 for details). T* will denote the conjugate operator of T. or(t), p(t) will denote the spectrum and the resolvent set respectively. For any x Q X and y * Y *, define the rank-one operator x y*gв (Y, X ) by the equation (а> У*)у=<У, 2/*X (V)2/G F. We recall a result which will be im portant for our purpose. Theorem IV of [3]. Let X, Y be Banach spaces, then T B ( X, Y ) has closed range i f and only i f у (27)> 0, where * y (T ) = inf{f[ 27a? [j, dist(a:, кегг) = 1} is called reduced minim um modulus. Let us begin with lemmas. Lemma I. Let X, Y be Banach spaces and А В ( Х ), B B (Y ). i) I f there exists A (7 such that R (A %) is not closed and there exist unit vectors 2/fcGFj y t^ Y * such that then у(^~лв) =0. (B -A ) 2/ft=lim (B -A )* 2/fc = 0, <yk, yt> = h, min Aft =A>0, ft-» oo ft ii) I f there exists A G xk X, xt G X* such that such that R (B L) is not closed and there exist unit vectors lim (A А)xk = (jd A)*2/ft = 0, (xk, xky = A^, m in Aj =Ao^>0>. ft-»oo ft then у(^~лв) =0. Proof Since ab= ^< a- wb->.), we may assume that A=0.. i) Since R (X ) is not closed, there exists a sequence {xk}renc X such that ' Define Then we have dist(a/fc, ker.a) = 1 > ajfc[ ИтЩ% =0., - to ft-»» J ^ к ~ а}к У*.

3 No. 3 Ъп, S. J. A NOTE ON GENERALIZED DERIVATION RANGES 263 \\A k- S kb\\=\\axk y*k- x k B*y*k \ < I Ахъ J + ^ ^ B*yt I >0, as b >oo. On the oilier hand, for any T 6 кет^~ав, we have $fc T\\ >\\ (Sk T )y k\ (viat kerbczker A ) >dist(v efc, ker A)>Xo>0, Therefore distos*,, кег^г"дв)>^о and hence у ( YAB) =0. ii) Since R(B*) is closed, there exists a sequence {у1}кевс:у* such th a t <Ust(s, U m M - O. Define Then we have W A J S ^ -S ^ B W ^ W A x ^ y t-^ B Y.i ^ (l+ I Axk I + 1B*yt [I >0, as Jo-*oo. On the other hand, for T квт^~дв we have II^-TII = ^ - Т Ц > (^ -Г )% 1 (via T*kre^d*czker B*)> d ist(xkyt, ker 5*) > ^ o > 0 Therefore d ist(8k, ker^~4b)>x o>0 and hence у(^~лв) =0. Lemma 2. Lei X, Y be Banach spaces and let A B ( X ), B B (Y ) be compact. I f one of the ranges B (A ) and B(B) is closed, while another is not closed, then R ( ^ ab) is not closed. Proof First we suppose В (A) is not closed and B{B) is closed. Then В is a finite dimensional operator and hence has the following m atrix form = B' on M + N = Y, where dimiv<oo. In this case, by Lemma IV. 2.3 and Theorem III of [3], there exist u n it vectors yk Y, yt Y * such that Bylc=B*yl=0, (yk, уч^ l. Thus у{^~лв) =0 results from i) of Lemma 1. The proof for the other case is similar. Definition, A sequence of vectors {uk}ken is called approximately linearly independent, i f for any n N, there exists m N such that i f Jcn>Jon-i> '>Ъ-у'>т, then any n vectors ukl,, ukn are linearly independent. Lemma 3. Let X, Y be Banach spaces and A B ( X ), B B (Y ) be compact. I f both B (A ) and B (B ) are not closed and there exist two sequences {vk}kenczy and {%k}k N'с О such that 1 ) {vk}ken is approximately linearly independent, ii) Bvk=Kkvk, then В (YTab) is not closed. '

4 264 CHIN. ANN. OF MATH. Vol. 6 Sor. В Proof By condition i), we may assume that (eventually discarding a finite number of v'ks)vti=0 and this fact will imply A7 lemma, we distinguish the following two subcases: In order to prove th is a) If As,=0 for infinitely many k s, then we have nulb=co. By the proof of Lemma 2 we have у(^~лв) =0. b) If As =0 for finitely many k's, by discarding those Afc we may suppose all of A» are not zero. Let M й, ivfc be the eigenspaces of A and В corresponding to {As} respectively (if Afc^<r(J.), then Жй = {0}). Since A and В are compact, we have dim Ж й<оо, dimivrfc<oo. By condition i) we may assume that AfcФ fact will im ply that Nni=Nw for 1оФ. Let yx Nx, yt Y * such that for Ъф and this and choose j/i F such that 1Ы М 2/?»==<2/^>=1, В*уЪ\>\В*У*х\\-1, 1-1. Let <na be a natural number such thab dim SP{N a,, N n,}> 2. By Lemma IV. 2.3 of [8], there exists a vector ya Sp{N a,, N n,} such that 12/2 I = dist («/a, S P{ylt By'x}) =1. Hence there exists a functional y\ Y* such that Ы \ = <2/a, t f > - 1, y te (Sp{yx, B y [})\ Continuing in the same way, we can construct three sequences, {«/ftbexcf, Ш ^ е ^ с Г such that yk.8p{ N N n*}, ^0=0, 1Ы =dist(j/», SP{y1} By\,, %fc-i}) =1, 2/fc =<3/fc, 2/fc>=l, < Й,В * Й > > В *!/: - -, 1Й 1-1. By the proof of Lemma 3.2 of [4] we may suppose jb*«/ ->0. Since R(A) is not dosed and dimm!e<oo) by Lemma 1.1 of [4] there exists a sequence {%}toe2fczx such that 03fc =dist(mfc, SP{MnH_1+1,, И % < -р Define Qk==xk yt We have I ^ abqhw-^- On the other hand, since 8М ъа М ь for S6ker^~4B, we derive that Thus the lemma is proved. dist(qfc, ker T AB) = in f{ Qfc- /S' j, ^ k e r ^ B} >ird{\\(qlt- S ) y li\\, S e k e v ^~AB} >dist(»j., 8р{Мщ_1+1,, Жя,} )= 1 > In the sequel, we shall consider the Banach space

5 No. 3 Lu, S. J. A NOTE ON GENERALIZED DERIVATION RANGES 265 Z ~ X + T, where the norm of the element z= (x, y) Z is defined as follows 11*1 =max{ a?fl, Jy }. ' We define the projections P, Q by the following equations Pz=*co, Q,z=y, for 2=25+2/, x X, y Y. Lemma 4. Let X, Y, Z, P, Q be defined as above. Let {Р»}»6лг{Щ к» с5 (2 ) be sequences o f projections with the following properties: i) { Pfcfl}fcetf is bounded, ii) И т Д > - Р, - 0, fc~»oo let {x]i}kenc z X, {pu}ic N, {zk}k Nc:Z be sequences such that iii) PjcPk^Pk, {Ы Ь еаг is bounded, iv) xk Ppk/!%! =1, dist( fe+i, SP{xi}hi) = lfo r h N, v) 2 Then for any n N and 0 < 8 < 1, there exists a natural number N (n, 8) such that provided hn+i> bn> " -> ls i^ N (n l 8). dist(p 2w Sp{Pzk} ni=1) >8, Proof We proceed by induction. Let n = l. Suppose the conclusion is false, then there exist two subsequences {zkl(n0}men, {zkt(m-)}menc-z such that d ist(p 2fcaCm), 8p{Pzkl<m)})< 8, as m->oo. Hence there exists a sequence {Ckl(m)}meNczO such that Since I I P Сыт)р %icx(m) I + 8, as m >GO' lim P2» = lim P ^ fcpfc = lim P P fc^ =lim % =1, /С-*оо к -* о k-*oо fc-*oо we have, ^ ( т ) < 1 + 5, as m->oo. By condition ii), we can derive 8 gs8 lim IP 2 ^'Jti(m)P^? 1(m) =Jdm P (E ki(n)pkt{m) 1 m~*<* = lm P(Pfta(m)P»a(m) ^'Sx(m)Pfci(m) >!ci(ro)) = hm жйа(т) ^ 1. tti >co m -*oo This contradiction shows that the conclusion is true for n = 1. Now assume that the conclusion is true for n 1 and it is not true for n. Then there exist sequences {zk(cm)}m&»(* 1,, n + l)czz, {OkiM}meN(i = l,, ft)c 0 such that I P zfc +,(m )@Ыт)Р%Ыт) -^fc (m)p«fc (m) <S, as m >oo. We shall prove that { Oknin() }я6у is bounded. Otherwise, we may assume that Then we have tffc (m) -*, as m->oo. '

6 268 CHIN. ANN. OF MATH. Vol. 6 Ser. В lim P2fcn(m) m-*» T his fact will imply that P^. =0. d is t (P2fc (m), $ P { P 2 fc((m)} "г/) >0, Which is a contradiction. Continuing in the same way we can prove in tu rn that (l0'fc -.(m) }mesr is bounded,, { Okl(m) }mewis bounded. Similarly, as in the proof for n = l, we can derive >hm[ P2fcn+1(m) ^fci(m)p^fcif m) *** C>Jc = Inn Cfc +1(nt) С'Ыт)а'Ыт) * 1^1. Again we obtain a contradiction and the proof is complete. Lemma 5. Let X, F Banach spices, A B ( X ), B B (Y ) be compact. I f both JR (A) and B (B ) are woi closed, then B (fy ab) is not closed. Proof By the symmetry, Lemma 5 is equivalent to the following: If both В (A) and В (В) are not closed, then B(fY~B&) is not closed. Therefore we need only to prove у (&~ab) =0 or у ($~ва) =0. By the proof of Lemma 3.2 of [4] we can construct four sequences : с:x*, {3/s}j=icF, {yj}"=icf*j such that. _ Ы - К 1 - Ы - '! Й И - 1, < fc, fc'> = <2/fe, y t'> S w for h<kf, lim[a fc[ =lim [ А*хЦ =И т[ Р^ [ =lim P*^ =0, fc->oo fc-> co fc-»oo fc-»oo where 8 ^ is the Kroneoker delta. Then we define operators Obviously, ^Тб ^ Уто) Рт( = Ук х/с. l i m ^ B17fc = l i m I ^ F fc = 0.. fc -» o o fc-»co. I f dist(z7fc, k e r ^ 4B) > 8 for some 8>0, then у(2гaf) ==0. If dist (F*,, ker *F~B4)> 8 for some 8>0, then у(^~вл) =0. Thus the lemma is proved. Therefore we assume fh at dist (Ufc, к е г ^ в ) = d ist(f ft, кег^вд) =0 and hence there exist two sequences -{ Afc} i c; ker.f~ib and { В ^ =1аЪ>т^~ВА such th at Consider the Banach space Define operators \Ub- A \ - * 0, IF» Pfc ->0. Z = X + Y. I t is easy to see that ran k TFfc=2, Ob ker AT, \\Wk~ Cft ->0,

7 No. 3 1м, 8. J. A NOTE ON GENERALIZED DERIVATION RANGES 267 Since spaces X and Y can be decomposed as follows where Х*.=кега, Ук=квту$>. Therefore Z can be written as where Z1c= X Jl+ Y lc. Put Z =Sp{%ic, УкУ+Zu, then pfc = ((g^l =1, { px, pis, }, {qi, q2, } are linearly independent respectively and WbPk =Pk, W-K4k = - 4k, WkZk = {0}. Hence, corresponding to the decomposition Z - S r ip d + S r fa b + Z t,, Wк has the following m atrix form Put /1 0 0 \ W k \0 0 0/ /1 0 o \ /о 0 0 \ = 0 0 0, Qfc-j \ / \ o 0 0 / Then Pk and Qk are rank-one projections and Wk~Pk~Qk. More precisely P k, Qk have the following forms P k ^lg Рк ( к + У}е), Qk= ~2 Чк (а'*с~у*с)) where <_<c+y, a%+yty = <p, ж*> + (у, yt>. Hence we have 1P*«-«Q * -1, PkPk=Pk, ЯкЧк^Чк, hen. By simple calculation, for к ф { 1, 0, 1} we derive that a - w. y ^ a - p. + Q ^ - ^ a - p ^ + Q ^ - P k ) - 1] -1 ^ ( l Pк) (1 Qk) Pk ^ Qk я 1 Я 1+Я Let Г be the circle with center Я=1, radius Г =-7Г and counter clockwise direction» A, Then we have On the other hand, for Я ^ { 1, 0, 1} we have Since (1 Pk) (1 ~~Qk) Я ldk X -O k= (X -W k) [l+(<k - W k ) - \ W k - o k)y, Pk I I Qk.1 Я II, I 1+Я j, Я -Г, 1, 2, is uniformly

8 268 OHIN. ANN. OF MATH. Vol. 6 Ser. В bounded and [ T F 0K >0, we may suppose Г а Р (О к). Put Obviously, E k is a projection and since Gk ker Av, we have E k ker A?. Moreover we can derive that д, - р, 1 o - c y - ' - о - ж * ) - 1! a < 2 г [ г 1 [1+ (».-ж 1) - 1(ж й- о у ] - > (х -ж,)-1 л. Hence we may assume that lim JP*~P*i =0. ' fc->oo Applying Lemma I of [3] we have rank 2?*.4-1. We now construct those three sequences defined in Lemma 3. Put Z-k^EbZ. Since dim Z*=l and Z k is an invariant subspace of T (i. e., T Z ^a Z f), there exists an eigenvalue Xk and for any vector z (E Zk>we have Tz =XjtZ. Put Zk=EkPk Since Zk^Zk, zk satisfies the above equation. Let гк=ик-\-гк, where щ X, Vk X. Then we have -Д-Щ= XfcMfc, Bvk=hjcV]C. We shall prove th at both {м*,}я,еу and {%}fcew are approximately linearly independent In fact, since {Pfc}fc6W, {Eb}h N, {xh}ken, {<pu}k N, {zk}k N satisfy the conditions of Lemma 4, for any n G 2V, there exist <5>0, m N suoh that for any Ckx, *., Okn 0,- II - OkUb ОкпщпI > 8, provided & +!>& >«* Passing to sequence {yk}k N and projection Q we obtain I ~ ОъРь Ch vkn I >8. Obviously, these facts will imply that {uk}k N, {vk}k N are approximately linearly independent. Thus the lemma follows from Lemma 3. Proof of Theorem The only if part of the theorem follows from Lemma 2 and Lemma 5, while the if part is a routine exercise, so we omit it. Thus the theorem is proved. References [ 1 ] Anderson, J. and Foias, 0., Properties which normal operators share with normal derivations and related operators. Pacific, J. Math., 61 (1975), [ 2 ] Fialkow, L A, Elements of spectral theory for generalized derivations, J. Operator theory, 3 (1980), [ 3 ] Kato, T., Perturbation theory fo r linear operators, Springer-Verlag, Berlin, [ 4 ] Lu Shijie, On the derivations with closed range in Banach spaces, OT6, , Birkhaiiser Veriag Basel 1982 (Proc. Conf. Operator Theory, Timisoara, Eomania, 6th, 1981,6,1 6, I I).